Disk drive memory systems store digital information in concentric tracks on a magnetic disk. The disk itself is rotatably mounted on a spindle, and information is accessed by means of read/write head located on a pivoting arm able to move radially over the surface of the disk. The radial movement of the head allows the different tracks to be accessed. Rotation of the disk allows the read/write head to access different sectors of the disk.
The spindle of a hard disk drive generally includes a spindle shaft that is interconnected to a hub that secures at least one disk. The disk is spun at a high rate of speed relative to the transducer head that reads and writes data from/to the disk. The hub and spindle are driven by an interaction of a magnetic field of a permanent magnet located on or otherwise associated with the hub and a magnetic field generated by a stator motor secured to a base plate of the disk drive housing. Stator coils are activated in a predetermined sequence to generate a variable magnetic field and drive the permanent magnet associated with the hub which initiates and sustains rotation of the disk.
Heads are designed to fly above the surface of the rotating disk. It is desirable to maintain a precise span or gap between the head and disk surface to achieve a low position error signal (PES) and thereby reduce read/write errors. As the head moves away from the disk surface, noise increases and read/write errors are more likely to occur. It is also critical that heads are able to maintain their position above intended tracks on a disk surface. Should a spindle move radially relative to its position as originally installed, particularly as track densities increase, errors can also occur in positioning the head relative to an intended track. Thus, the spindle shaft must be held in a predetermined and fixed orientation in order to minimize spindle tilt and associated disk flutter or wobble which varies the gap between the head and disk surface and in order to minimize radial movement or drift of the spindle. Both of these problems may be addressed by increased spindle stiffness.
Spindle stiffness is often accomplished by the utilization of a bearing sleeve. The sleeve is positioned around the spindle shaft and includes at least one journal bearing that interfaces directly with the spindle shaft through an intermediate lubricating fluid. In order to ensure that the fluid is positioned between the journal bearing and the spindle shaft and to provide enhanced stiffness, grooves may be employed on bearing surface as taught in U.S. Pat. No. 6,313,967, which is incorporated by reference in its entirety herein. Journal bearings increase the radial stiffness of the spindle shaft, restricting radial movement of the spindle in a direction perpendicular to the axis rotation of disks. Journal bearings also substantially prevent tilting of the spindle shaft relative to the spin axis of the spindle and disk.
Bearing sleeves of the prior art generally include an upper journal bearing surface and a lower journal bearing surface wherein the distance between the centers of the two bearing surfaces equals the total bearing span. The greater the total bearing span, the greater the pitch stiffness provided to the spindle. Bearing stiffness can also be increased by reducing the space or gap between the sleeve and the spindle shaft. Reduction in radial gap, however, leads to an increase in bearing drag and thus an increase in the power required to maintain a predetermined spin rate of the disks.
Another way to increase bearing stiffness is to apply axial loading to the spindle. One method of accomplishing this is to apply a predetermined amount of compression to the spindle upon installation and forcing a bearing surface associated with the spindle against a thrust plate disposed opposite the spindle bearing surface, typically at one end of the spindle. Utilizing a double thrust bearing, which incorporates a thrust plate at both ends of the spindle shafts, further enhances stiffness. Another method to enhance stiffness is to magnetically induce an axial load on the hub that presses the spindle shaft onto a thrust bearing that is located at the end of the spindle shaft and beneath the hub. A magnetic bias plate is used to draw a magnet disposed on the hub downward and thereby force the spindle shaft onto the thrust bearing. Disk drives that utilize thrust bearings have the disadvantage of having to accommodate the vertical height of the thrust bearing. Increased height may limit the applicability of using the disk drive in small electrical devices where vertical or Z height is a factor. Furthermore, even though a double thrust bearing system often used in the art can be more robust than a single thrust bearing, it does not provide sufficient stiffness in a disk drive using 2.5 inch diameter or smaller media.
Thus, it is a long felt need in the field of magnetic disk drive manufacturing to provide increased bearing stiffness to counteract radial and pitch loading while allowing for reduction in disk drive height. The following specification describes a system that employs a multipurpose bearing that substantially combines the functions of the thrust and journal bearings to enhance stiffness, while simultaneously counteracting radial and pitch loads on the shaft. As a result, the thrust bearing traditionally used in prior art disk drives may be eliminated to allow for the disk drive to be reduced in height without sacrificing spindle stiffness. Embodiments of the present invention optionally permit the elimination of the magnetic bias plate which reduces magnetic noise and cost.